drop_off
1. [ verb ] fall or diminish
Examples:

"The number of students in this course dropped off after the first test"

Related terms: decrease decrease
2. [ verb ] change from a waking to a sleeping state
Synonyms: fall_asleep drift_off flake_out nod_off doze_off drowse_off dope_off
Examples:

"he always falls asleep during lectures"

Related terms: wake_up zonk_out
3. [ verb ] retreat
Synonyms: lose fall_behind fall_back recede
Related terms: gain regress recession
4. [ verb ] get worse
Synonyms: slip fall_away drop_away
Examples:

"My grades are slipping"

Related terms: worsen lapse slump
Similar spelling:   drop-off
  drive_off
  drop_open